In the manipulation of machinery, particularly farm implements, it is often necessary to hydraulically raise and lower the machinery repeatedly and reliably to preset positions. In the case of farm equipment, for example, it may be necessary for a tractor operator to lower a plow or other implement to a particular position so as to plow land to a desired depth. This depth may change according to varying requirements furrow to furrow or field to field and it is therefore necessary that the operator have the ability to change this depth with ease. Furthermore, it may be necessary to frequently raise the implement off the ground, for example when turning at the end of the field or for maintenance, and then return the implement to the proper depth setting.
In the prior art, the typical system used for this task includes a hydraulic cylinder connected between the implement frame, which is suspended over the ground with wheels, and the implement tool which is pivotally mounted to the frame. Thus, the relative positions of the frame and tool may be varied hydraulically utilizing standard tractor hydraulic systems. While in this manner an implement tool may be raised and lowered, it cannot be accurately positioned nor reliably maintained in a desired position due to, among other things, leaky valves and hoses and the high volume fluid flow rates typical of tractor systems, which make fine adjustments in piston position extremely difficult. Accordingly, devices independent of the standard control valves provided on the tractor have been employed in connection with the cylinders to enable an operator to accurately set a desired piston position.
One simple device used for this purpose is a ring which may be clamped around the piston rod to stop the stroke and thereby the implement tool at the desired working position. Another relatively simple device utilized is a poppet valve mounted on the cylinder and actuated by means attached to the rod to shut off hydraulic fluid flow to an appropriate cylinder chamber and stop the stroke when the working position is reached. Neither of these devices, however, can be remotely reset or adjusted and therefore the operator must stop the tractor and do so manually. If all fields were table top flat and of consistent soil composition, these systems would be somewhat adequate. Unfortunately, grade and composition often vary from one end of the field to the other, and to achieve uniform optimum implement penetration requires repeated manual readjustments.
Obviously, optimum soil penetration or working depth is often sacrificed for speed when stroke limiting systems of the above described type are employed. The result is reduced yield and unnecessary erosion, which is perhaps the foremost problem of today's farming industry. Therefore, it may be seen that better control of penetration depth can more than relieve inconvenience or inefficiency of equipment operation, but can provide improvements in both yield maximization and soil conservation. Thus, efforts have been and continue to be made to develop depth control systems in which, at least, depth settings may be manually controlled or adjusted from the tractor cab while on the go.
The above-described mechanical or manually adjusted systems offer the ability to control the relative positions of the implement frame and implement tool. However, the more important aspect of tillage implement control relates to the desirability of controlling the "actual" soil penetration depth of the implement tool, which as one skilled in the art knows, is not the same as controlling the relative positions of the implement frame and tool. For example, when passing from hard to soft soil the wheels of the frame sink further into the ground, and so too does the tool. Therefore, it is necessary to adjust the relative position of the frame and tool to position the tool at the desired depth. Thus, it will be seen that presetting the stop or working position of the piston only guarantees the tool's position relative to the frame and does not guarantee the actual position of the tool relative to the earth's surface. Accordingly, depth sensors have been developed to monitor the actual depth of the tool and provide a signal to the cab of the tractor so that the operator may at least stop and readjust the relative positions of the tool and frame and accordingly the depth of the tool's penetration. While this method of control provides a means to improve uniformity of tillage it nevertheless requires the operator to pay close attention to the monitor signal and manually correct deviations from the desired position. However, depth sensors have provided a vehicle to permit automation of depth control.
The conventional automatic system in use today consists of a single-piston and cylinder arrangement employing feedback from the depth sensor to control solenoid actuated valves for directing fluid into and out of the cylinder chambers. In these systems the desired working depth is preset remotely (for example in the cab) via an electrical circuit, the setting of which is constantly compared against the actual depth of the implement tool to generate an adjustment signal which controls and hydraulic fluid flow to the cylinder. Typically, proportional or "analog" feedback is employed so that the magnitude of adjustment fluid flow is commensurate with the degree of adjustment required. However, systems of this nature typically lack the measure of reliability and repairability that farming demands. More specifically, most, if not all of these systems utilize precision variable flow valves such as flow dividers to control the flow of fluid to the cylinder. While satisfactorily operative in the laboratory where their position may be accurately controlled, they are notoriously unreliable in the field, in which they are often subjected to harsh environmental conditions which can easily degrade or interrupt their operation. Furthermore, they are expensive and relatively difficult to replace or repair, and to some extent suffer from susceptability to overheating, as they depend on constant readjustments to compensate for leakage. Due to the remote location of most farms, it is not now uncommon for a farmer to lose one or more days of precious time waiting for the skilled technicians often needed to effect repairs.
Thus, notwithstanding the best efforts of many, there remains a need for more reliable depth control systems for tillage implements. As will be seen from the following the present invention provides a relatively simple and inexpensive automatic actual-depth control system for tillage implements characterized by high reliability and ease of repairability.